Great Teacher Onizuka was the first manga I read, but it wouldn't be for another year that I started actively collecting them.

Ai Yori Aoshi and Excel Saga are what really got me hooked. I completed Ai Yori Aoshi, and I still have several volumes of Excel Saga to buy before I'm current with the series (and I have to do that soon, the older volumes are getting scarce)

Though lately I'm getting hooked on Light novels. Mainly Spice and Wolf. I can't stop reading it. I really enjoyed the anime, and reading it gives me a whole new perspective on the story. I actually think I like the novel a lot more.